Variable inductance structure



July 14, 1953 E. o. KEIZER VARIABLE INDUCTANCE STRUCTURE 2 Sheets-Sheet l Filed June 6, 1950 lNVENTOI? Eugene Kezzer ATTORN E July 14, 1953 lE. o. KEIZER VARIABLE INDUCTANCE STRUCTURE 2 Sheets-Sheet 2 Filed June 6, 1950 lNVENTOR. Eugene Q /fvezzel A'ILTORNEY Patented July 14, 1953 UNITED VARIABLE INDUCTANCE STRUCTURE Eugene O. Keizer, Princeton, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application June 6, 1950, Serial No. 166,428

(Cl. Z50- 40) 6 Claims.

This invention relates to Variable inductance structures and more particularly to a variable inductor suitable to provide a high-frequency tuner for radio and other electrical circuits and tunable over an extended high-frequency range.

At the present time, several channels 'have been allocated for the purpose of broadcasting television images in black and white. These television channels are within the frequency bands from 54 to 88 mc. (megacycles) and from 174 to 216 mc. Thus, for the reception of such television images these two widely separated frequencyranges must be covered by a high-frequency .tuner forming part of a television receiver. It is expected that in the future tele- Vision images will be transmitted over a still higher frequency range. At the present time, the frequency -band between 490 and 700 mc. has been reserved for experimental purposes in connection with color television. It is accordingly desirable to provide a single high-frequency tuner which will cover, for example, these three widely spaced high-frequency ranges. Such a highfrequency tuner may be designed to operate in the .manner of a spread-band tuner whereby a desired frequency range is expanded or spread out while intermediate, undesired frequency ranges are compressed. The present invention provides a variable inductance structure which willpermit tuning of such widely spaced frequency `ranges between .approximately 50 and more than y'700 mc.

It is accordingly an object of the present invention to `provide a variable inductance structure for varying the tuning response of an electrical circuit over a wide frequency range such as, for example, between approximately 50 and over 700 mc.

A further object of the invention is to provide a variable inductance structure where equal movements of a movable element will result in either a rapid or a slow variation of the inductance of the structure whereby the structure may be used in an electrical circuit to spread out a desired frequency range while an undesired frequency range may be compressed.

Another object of the invention is to provide a variable inductance structure having terminals arranged in such a manner that the structure may be connected, for example, to a vacuum tube or to other circuit elements by very short leads, thereby to reduce stray capacitance and inductance which may be present at high frequencies when long connecting leads have to be used.

A variable inductance structure for varying the tuning response of an electrical circuit over a relatively wide high-frequency range in accordance with the invention comprises a pair of inductors. The inductance of sections of equal length of each inductor varies in a predetermined manner along the length of the inductor. A single metallic slider is disposed about a portion of each inductor and is spaced therefrom by a dielectric layer which may, for example, be provided on each inductor. Thus, a capacitance is provided between the slider and each of the inductors. The slider is movable with respect to the inductors to vary the inductance existing between one end of each inductor and the slider. Electrical terminals may be connected to one end of each inductor for connection to an electrical circuit. Thus, if a capacitance is provided between the two terminals, a parallel resonant circuit is obtained, the resonant frequency of which may be varied by movement of the slider.

The novel features that are considered characteristic of this invention are set forth with particularity in the appended claims. The invention itself, however, both as to its organization and method of operation, as well as additional objects and advantages thereof, will best be understood from the following description when read in connection with the accompanying drawing, in which:

Eigure .1 is an elevational View, partly in section, of a variable inductance structure embodying the present invention and suitable to provide a high-frequency tuner;

Figure 2 is a cross-sectional view of the structure of Figure 1 takenon line2-2 of Figure 1;

Figure 3 is a sectional view of a modified `inductance structure in accordance with the invention;

Figure 4 is an equivalent circuit diagram of the inductance structure of Figure 1 which is shown connected to an oscillation generator;

Figure 5 vis a graph showing curves illustrating by way of example the relation between the resonant frequency of a structure in accordance with Figures 1 to 4 with respect to the motion of a movable slider thereof; and

'Figure 6 is a perspective View of the tuner shown in Figures 1 and 2 for more clearly illustrating the relationship of the various elements of the invention.

Referring now to the drawing, in which like components are designated by the same reference numerals, and particularly to Figures l, 2 and 6 there is illustrated a variable inductance structure 26 in accordance with the invention. The variable structure includes a pair of inductors generally indicated at IG and (Figures 2 and 6). The two inductors In and II are of the same construction and one of them is shown in detail in Figure 1. Thus, inductor I includes a metallic rod I3 which may be solid and which may be tapered towards its free end as shown. Terminal I4 is electrically connected to the smaller end of rod I3.

A hollow support |5 is mechanically secured to rod I3. Support I5 consists of a suitable insulating material. A metallic ribbon I6 is disposed spirally about support I5 and is electrically connected to rod I3, for example, by solder indicated at I'I. Preferably, the width and pitch of ribbon I5 is largest near rod I3 and decreases towards its free end at the right of Figure 1. A dielectric layer I8 may be disposed about ribbon I5 and rod I3. Layer |8 may, for example, consist of polystyrene. A core of a suitable ferromagnetic material such, for example, as a powdered iron core may be disposed within support |5. A similar core I 9 may be provided in inductor I I (Figure 2). Core 20 serves to increase the inductance of a particular portion of inductor I0. It is to be understood that more than one core 20 may be provided, if desired.

A metallic slider or bridge element 2| is provided which surrounds a short axial length of both inductors I0 and II. Slider 2| is movable with respect to inductors I0 and Il as indicated by arrow 22. A capacitive pickup element 23 of a radio signal may be disposed about inductor I0. For some applications `such as, for example, a push-pull mixer circuit a separate pickup element may be coupled to each inductor III and It is also feasible to provide a cylindrical portion 24 which is secured to slider 22 and which extends between the slider and the free end of each inductor lll and II. Cylinders 24 may, for example, be made of a high-loss magnetic material such as magnetite or ferrite which have a comparatively high magnetic loss at frequencies of 50 mc. or more. Instead of providing a dielectric layer I8 about core |3 and ribbon I6, such a layer may be provided in the openings of slider 2 I.

The operation of the structure of Figures l, 2 and 6 will now be evident. The position of slider 2| determines the inductance existing between the slider and terminals I4. The rate of change of the inductance with the movement of slider 2| is determined by the pitch and width of ribbon I6. This relationship may be adjusted by proper positioning of ferromagnetic core 20 or by providing more than one core within support I5. Cylinders 24 serve the purpose of rendering ineffective the portions of inductors I0 and II existing between slider 2| and the free ends of the inductors. It will also be understood that the inductance of the structure is smallest when slider 2| is moved to the left and increases when the slider is moved towards the right.

In order to render effective the unused portions of inductors I0 and which exist between the slider 2| and the free ends of the inductors it is also feasible to utilize the structure illus trated in Figure 3. Cylinder or lshell 24 completely surrounds inductor I0 and may also surround inductor and preferably has a closed end portion indicated at 50. Cylinder 24 again consists of a material having high magnetic loss such as ferrite, magnetite or Bakelite. Cylinder 24 is connected to slider 2| and moves in unison therewith. A solid core 5I may be connected to end portion 50 and also consists of a material having high magnetic loss. The free end of core 5| is spaced from slider 2|. It will therefore readily be seen that core 5| and shell 24 will damp out the unwanted resonant frequencies which may exist in the unused or free portions of inductors land Figure 4 illustrates an oscillator generator wherein dotted box 26 indicates diagrammatically the structure of Figure 1. Variable inductors l0 and II correspond to those of Figures 1 and 2. Capacitors 2l and 28 represent the capacitance between slider 2| and inductors I0 and |I respectively. The capacitors 21, 28 have been shown variable because these capacitances will decrease when `slider 2| moves over tapered rod |3.

The variable inductance structure 26 shown in Figure 4 forms part of an oscillation generator including triode 30 having a cathode 3|, control grid 32 and an anode 33. Cathode 3| is grounded through choke coil 34 and bias network 35 consisting of a capacitor land a resistor connected in parallel. Control grid 33 is grounded through a grid leak resistor 36. Anode 33 is connected to a suitable voltage supply indicated at -I-B through choke coil 31. Inductors IU and are connected respectively to anode 33 and control grid 32.

The oscillator of Figure 4 may be considered to operate in the manner of a Colpitts oscillator. Capacitor shown in dotted lines between anode 33 and control grid 32 indicates the inter-electrode capacitances of the tube. Thus, the capacitor 40 includes the anode-grid capacitance, the anode-cathode capacitance as Well as the gridcathode capacitance of tube 30. Inductance structure 2E in combination with capacitor 40 represent a parallel resonant circuit. Its resonant frequency is determined by the position of slider 2| in the manner previously described.

The oscillatory output wave may be derived, for example, from inductor 4| inductively coupled to choke coil 34 in the cathode circuit. Alternatively, the output Wave may be derived from anode 33 through output terminals 42, one of which is grounded while the other one is coupled to the anode through coupling capacitor 43. It is, of course, also feasible to derive an output wave from capacitive pickup 23 as `shown in Figures 1 and 2.

Figure 5 illustrates curves 45 illustrating the frequency of the oscillator of Figure 4 with respect to the motion of slider 2| from terminals |4. The frequency range is between approximately and over TOO mc. During a portion of the undesired frequency range the oscillator does not oscillate due to resonances in coils 31 and 34. The heavy portions 4B, 4'I and 48 on curves 45 indicate the desired frequency ranges. Thus, if the oscillator of Figure 4 is used in a superheterodyne receiver and if the intermediate frequency is 25 mc., the oscillator range should be 79 to 113, 199 to 241 and 515 to 725 mc. as shown in Figure 5 in order to cover the three television bands 54 to 88, 174 to 216 and 490 to '700 mc. Inductors I0 and II may have a length of between 4 and 6 inches and an inner diameter of onequarter inch to cover this frequency range.

The oscillator of Figure 4 has the advantage that it is not necessary to provide a physical capacitor between the terminals I4, that is, be-

tween anode 33 and grid 32. A further advantage is thatlthelterminals may be-'brought out close together so th'at'ithe leadsiwhichccnnect the inductance structure to tube 30.rnay be made very short. This, in turn, reduces stray inductance and'stray capacitance. The inductance range .is extremely wide and the over-all 'length of inductance l0-and II may be very short. This is dueto the fact that the full length of the inductors is utilized. for'the low-frequency end of the tuning range corresponding to the high inductance of the structure. At the high frequency end of the tuning range the capacitance of capacitors 21, 23 is reduced, thereby to increase the effective tuning range. Furthermore, no shunt capacity exists between terminals I A so that the structure of the invention may readily be used with other high-frequency circuits.

It is to be understood that rods I3 may be omitted in which case terminals I4 would be connected directly to ribbons I6. Cylinders 24 may also be omitted, if desired.

What is claimed is:

l. A variable resonant structure for varying the tuning response of an electrical circuit cornprising a pair of inductors, each having a substantially straight support of insulating material, a metallic helical ribbon disposed about each of said supports, each of said ribbons having a pitch Varying along its length in a predetermined manner, a dielectric layer disposed about each of said ribbons, a terminal connected to one end of each of said ribbons, a metallic slider surrounding a short portion of the axial length of each of said inductors and spaced therefrom by said dielectric layer to form a capacitance between each inductor and said slider, said slider being movable along said inductors to vary the inductance existing between said terminals and said slider, a high loss element disposed about each of said layers, said elements being disposed between the other end of each of said inductors and said slider and being secured to said slider, a radio signal pickup element coupled to one of said inductors, and capacitive means connected between said terminals.

2. A Variable inductance structure for varying the tuning response of an electrical circuit comprising a pair of inductors, each having a substantially straight support of insulating material, a metallic ribbon disposed spirally about each support, a metallic rod one end of which is connected to each of said ribbons, a terminal secured to the other end of each of said rods, said rods being tapered toward said terminals, each of said ribbons having a pitch varying alongr its length in a predetermined manner, a dielectric layer disposed about each ribbon, a single metallic slider completely enclosing a short axial length of each of said layers to form a capacitance between said slider and each of said inductors, said slider being movable with respect to said inductors to vary the inductance existing between said terminals and said slider and wherein the capacitance between said slider and said rods is varied when said slider moves over said rods, circuit connections coupled to said terminals, and a radio signal pickup element coupled to at least one of said inductors.

3. A variable inductance structure for varying the tuning response of an electrical circuit comprising a pair of inductors, each consisting of a metallic ribbon disposed spirally about a cylindrical surface, a terminal secured to one end ofeach oi said ribbons,.each of saidribbons having a ywidth and a pitch which vis .largest adjacentits terminal and which decreases towards its free.end,..a dielectric layer disposed aboutl each of said ribbons, a single metallic slider enclosing a'short axial lengthof each of said inductors to forma capacitance between said slider and each ofisaid inductors, said slider being movable with respect to said inductors to vary the inductance existing betweensaid terminals and said slider, a shell surrounding the free end portion of each of said ribbon and secured to said slider, and a core of high-loss magnetic material secured to said shell and disposed within said free end portion of each of said ribbons, thereby to damp out unwanted resonant frequencies which may exist in said free end portions.

4. A variable inductance structure for varying the tuning response of an electrical circuit comprising a pair of inductors, each consisting of a metallic ribbon disposed spirally about a cylindrical surface, a terminal secured to one end of each of said ribbons, each of said ribbons having a width and a pitch which is largest adjacent its terminal and which decreases towards its free end, a dielectric layer disposed about each of said ribbons, a single metallic slider enclosing a short axial length of each of said inductors to form a capacitance between said slider and each of said inductors, said slider being movable with respect to said inductors to vary the inductance existing between said terminals and said slider, a shell surrounding the free end portion of each of said ribbons, said shell having a closed end portion and being secured to said slider and movable therewith, said shell consisting of a high magnetic loss material, and a core of high-loss magnetic material secured to said shell and disposed within said free end portion of each of said ribbons, thereby to damp out unwanted resonant frequencies which may exist in said free end portions.

5. A variable inductance structure for varying the tuning response of an electrical circuit, comprising a pair of inductors each having a substantially straight support of insulating material, a metallic ribbon disposed spirally about each support, a terminal secured to one end of each of said ribbons, each of said ribbons having a width and a pitch which is largest near its terminal and which decreases towards its other end, a dielectric layer disposed about each of said ribbons, a single metallic slider completely enclosing a short axial length of each of said layers to form a capacitance between said slider and each of said inductors, said slider being movable with respect to said inductors to vary the inductance existing between said terminals and said slider, a high loss element disposed about each of said layers, said elements being disposed between the other end of each of said inductors and said slider and being secured to said slider, and circuit connections coupled to said terminals.

6. A variable inductance structure for varying the tuning response of an electrical circuit comprising a pair of inductors, each consisting of a metallic rod and of a metallic ribbon electrically connected to said rod and disposed spirally about a predetermined elongated surface, each of said ribbons having a width and a pitch which is largest adjacent said rod and which decreases towards its free end, a terminal secured to the free end of each of said rods, each of said rods being tapered toward said terminals, a dielectric layer disposed about each of said rods and about '7 each of said ribbons, a single metallic slider enclosing a short axial length of each of said inductors to form a capacitance between said slider and each of said inductors, said slidel` being movable with respect to said inductors and to said rods to vary the inductance existing between said terminals and said slider and the capacitance between said slider and said rods, and an electrical circuit connected to said terminals.

EUGENE O'. KEIZER.

References Cited in the file 0f this patent UNITED STATES PATENTS Number Nam-e Date Wheeler et al Nov. 19, 1940 Schaper Dec. 7, 1943 Van Der Heem Nov. 21, 1944 Beetham July l5, 1947 Guanella Aug. 2, 1949 

